Cerrosafe is for chamber casts, not slugging the bore.
I have one 1896 infantryrifle with groove diameter .3125". I also have one .3075", two (or 3 I forget) that are right around .310. My first one (found in a flower shop (!) in Worcester circa 1969) is .3087. I have one sporter that has a M1917 barrel (5 groove LH twist)on it, .311 groove (I have the correct mike for 5 groove slugs).
Moral of the story: you really need to examine and measure your bore to maximize the accuracy potential. Another jacketed bullet to consider is the Lapua D46 bullet for 7.62x 54. 200 grain rebated boat tail, .309". Not cheap, but will make a credible 600 yard shooter of an oversized (if unabused) bore.
USN Distinguished Marksman No. O-067